The present invention relates to the production of intermediates to make agents useful for the production of light by chemiluminescence.
The use of 3,5,6-trichlorosalicylic acid as an intermediate in the preparation of n-pentyl 3,5,6-trichlorosalicylate was described by Rauhut in Example XXVI of U.S. Pat. No. 3,749,679. The latter compound is an intermediate in the preparation of bis(2,4,5-trichloro-6-carbopentoxyphenyl)oxalate which is useful in chemiluminescent compositions, as described in U.S. Pat. Nos. 3,597,362; 3,781,329; and 3,816,326.
Rauhut also discloses in Example XXVI of U.S. Pat. No. 3,749,679 the conversion of 3,5,6-trichlorosalicylic acid to the n-pentyl ester by refluxing it in n-pentyl alcohol containing a catalytic amount of sulfuric acid. The crude product is obtained in a yield of about 73% of theoretical. The combination of Hanna, U.S. Pat. No. 3,062,877, Leulier and Pinet, Bull. Soc. Chim. 41, 1362-1370 (1927), and Rauhut indicates an overall yield for the preparation of the n-pentyl ester, starting with salicylic acid and chlorinating, of about 53% of theoretical.
Manfre, Mohan and Rauhut disclose in U.S. Pat. No. 4,308,395 processes that produce significantly higher yields of 3,5,6-trichlorosalicylic acid and esters thereof, starting with salicylic acid. In accordance with that invention, salicylic acid is treated with gaseous chlorine in concentrated sulfuric acid to form 3,5-dichlorosalicylic acid. This product is converted by further chlorination in oleum, containing iodine, at about 40.degree.-60.degree. C., to 3,5,6-trichlorosalicylic acid, which is recovered by drowning the reaction mixture in ice and water, whereupon the product precipitates as a solid. The 3,5,6-trichlorosalicylic acid precipitate can be extracted from the drowned reaction mixture by mixing in a water-immiscible aromatic hydrocarbon solvent at a temperature of at least 60.degree. C., and separating the organic phase, which contains the 3,5,6-trichlorosalicylic acid. Xylene is a preferred immiscible solvent for this purpose. The process is also applicable in the preparation of esters of 3,5,6-trichlorosalicylic acid represented by formula (I) ##STR1## in which R represents alkyl (C.sub.3 -C.sub.20). A solution of 3,5,6-trichlorosalicylic acid is prepared as described above, by extracting the drowned reaction mixture with a mixture of water-immiscible aromatic hydrocarbon solvent and an alcohol (C.sub.3 -C.sub.20). This mixture is treated with a catalytic amount of a strong acid, such as sulfuric acid, p-toluenesulfonic acid, and the like, and heated at an elevated temperature to evaporate the water-alcohol azeotrope from the solution which contains the compound of formula (I), in the immiscible organic solvent.
In one especially preferred embodiment, the water-immiscible hydrocarbon solvent is xylene, the extraction is at 75.degree.-100.degree. C., the alcohol is n-pentanol, and the acid catalyst is concentrated sulfuric acid.
In practice, it has been found that such a process has at least one drawback; namely, that a large amount of dipentyl ether is formed as a byproduct. The large excess of pentanol required decreases the product yield per batch.
It has now been discovered that titanium esters and chelates, used in place of acid catalysts, selectively catalyze esterification, to the virtual exclusion of etherification. Moreover, the loading of pentanol needed drops considerably. Finally, the process has been found to be broadly applicable to esterification of this acid with other alcohols, even if the technique of Manfre et al is not used.
Some advantages of the present invention are as follows:
1. The esters of 3,5,6-trichlorosalicylic acid are obtained in very high purity, and in a relatively high yield.
2. The esters of 3,5,6-trichlorosalicylic acid can be prepared directly from salicylic acid by a series of steps which can all be performed in a single reaction vessel.
3. The esters of 3,5,6-trichlorosalicylic acid can also be obtained in a very high yield and purity, without the need for isolating the 3,5,6-trichlorosalicylic acid. The overall yield from salicylic acid to n-pentyl 3,5,6-trichlorosalicylate is excellent and the purity of the product is higher than about 98%.
4. The elimination of ether byproduct formation allows the reduction of the alcohol charge from 4 eq. to 1.75 eq. or less. Thus, a higher charge of the 3,5,6-trichlorosalicylic acid is allowed per campaign, and it is no longer necessary to remove and discard the byproduct as waste.